The invention arose during continuing development efforts in electric circuit protection techniques, including residential, commercial and industrial applications having overcurrent protection systems for preventing personal injury and property damage. Such overcurrent protection systems typically include a load center or panelboard receiving electrical power from the utility company transformer, and routing the electrical power through a main circuit breaker or fuse and then through branch circuit breakers or fuses to designated branch circuits, each supplying current to one or more electrical loads. The circuit breakers or fuses are designed to interrupt the electrical current if it is excessive or outside the design limits of the conductor and loads, to reduce the risk of injury and damage, including fires. Circuit breakers are more commonly preferred because they are reusable simply by resetting same.
A circuit breaker has a thermal/magnetic trip characteristic. The thermal characteristic is operative in response to overload current of extended duration which heats a bimetal member, causing movement of the latter, which in turn releases a latch to trip open a set of contacts. For example, the thermal characteristic would respond to 30 amps being drawn in a 15 amp circuit. The magnetic characteristic is operative in response to a sudden high magnitude current overload condition, and uses the magnetic field generated in a magnetic core to attract an armature, which movement releases the latch to open the contacts. As an example, the magnetic type actuation occurs in response to a short circuit wherein the hot line conductor becomes directly connected with ground or neutral, bypassing the load.
A further type of circuit protection is provided by ground fault interrupters, which trip the breaker to an open circuit condition in response to an imbalance of currents between the line and neutral conductors of a given circuit. .This is particularly desirable in the event that a person is the path to ground.
The present invention addresses a further type of electric circuit protection and monitoring, namely the detection of arcing in the electric circuit. The arcs to be monitored include high impedance faults or other intended or unintended circuit paths which do not have sufficient energy or current flow to jump a gap or trip a breaker, but nevertheless can generate damaging heat or other undesirable effects. An arc may occur in series or parallel with the load, and may have numerous causes, for example loose connections, worn or aged wiring or insulation, prior electrical stressing such as caused by overuse, prior lightning strikes, etc. The current in such arcs may be below the thermal and magnetic trip settings of the breaker or a fuse, and/or may not present an imbalance of currents between the line and neutral conductors, and hence such arcs can go undetected. A particular hazard due to arcing is hot spots along electrical wiring in a residence or the like, which is a fire risk.